A method of converting an output voltage of a solar cell with an inverter and supplying this converted power to a pump apparatus or the like allows feeding of water, irrigation, and the like to be performed even in regions in which stable electric power cannot easily be supplied, such as intermontane regions, and thus is highly useful for such regions. In such regions, it can be considered that water may be pumped up from a well or the like with use of an engine pump or the like. However, the engine pump requires fuel to be supplied thereto, which is inconvenient. Specifically, in such a system using an engine pump or the like, if supply of fuel is stopped, then it may be considered that feeding of water is also stopped. In contrast thereto, a system utilizing a solar cell as an energy source does not need fuel to be supplied thereto, and is highly convenient because water can be pumped up as long as sunlight is applied to the solar cell.
This type of water supply apparatus comprises a solar cell for converting sunlight into electric energy, an inverter for converting direct-current power supplied from the solar cell into alternate-current power suitable for operation of a pump, a motor for driving a rotatable shaft while being supplied with electricity from the inverter, and a pump driven by the motor. Generally, a motor pump for pumping up water is disposed at a bottom of a deep well or the like to pump up water, and the pumped water is stored in a tank on the ground. Such a pump utilizing a solar cell is operated by electric power produced in accordance with an amount of solar radiation. Since the pumped water is stored in the tank, the water can be used as needed.
A submerged pump is generally used as the pump in the above system, a three-phase induction motor is generally used as the motor for driving the pump, and an inverter supplies alternating-current power to the motor. Electric energy supplied from the solar cell varies depending on the amount of solar radiation and operating conditions of the motor pump (e.g., voltage, current, and frequency). Therefore, a maximum electric power tracking control in which the voltage, the current, and the frequency are controlled so as to supply the maximum electric power to the pump is performed to operate the pump most efficiently.
The following are required for such a water supply apparatus utilizing a solar cell. First, in order to utilize solar energy efficiently, it is necessary to maximize efficiency of the entire system. Further, since the pump is disposed within a well or the like, the pump needs to be small and lightweight, to be strong, and to have little trouble. The pump should be capable of being easily handled so that an operator who operates the pump can operate the pump with ease. Furthermore, it is necessary to protect the pump sufficiently, for example, to prevent the pump from racing due to water shortage in the well, or to output an alarm in advance and stop the pump when trouble is detected which would cause the pump to be damaged.
In order to enhance efficiency of a motor pump, it has heretofore been considered that a high-efficiency DC brushless motor should be used as the motor in the motor pump. A DC brushless motor controls currents supplied from an inverter to windings by switching the currents according to a rotational angle of a rotatable shaft. Specifically, the DC brushless motor supplies the currents to the windings of the motor sequentially in accordance with a detected rotational angle of the rotatable shaft to thereby rotate the rotatable shaft. Generally, the rotational angle of the rotatable shaft is detected with use of a magnet fixed to a portion of the rotatable shaft and a position sensor such as a Hall element for detecting the position of the magnet. Thus, it is necessary to provide the position sensor for detecting the rotational angle of the rotatable shaft, a sensor circuit with the sensor, sensor wires for transmitting the rotational angle of the rotatable shaft to the inverter, and the like.
However, since the motor pump is disposed at the bottom of a well or the like, as described above, a system using a sensor such as a Hall element is not suitable for a submerged pump installed in a well because of the increased number of wires. Further, when the number of parts, such as a sensor element and a sensor amplifier, increases, a possibility of trouble increases accordingly, resulting in necessity of maintenance. In order not to expose such sensor wires to an exterior, the sensor wires and the inverter may be installed in a casing of the motor. However, if the inverter is disposed in the casing of the motor, then space is required for the motor itself and a structure of the motor pump itself becomes complicated. Thus, maintenance burdens become greater.
Further, such a water supply apparatus requires a controller for controlling starting and stopping of the motor pump and outputting signals to an external device. Since an inverter generally includes such a controller therein, it is necessary to separate the controller and the inverter from each other, which is inconvenient. Further, it is feared that water may enter an interior of the motor, and, if the inverter is not installed in a good environment, then it becomes necessary to pull up the pump from the well in order to handle any trouble with the inverter. Thus, many problems arise in view of maintenance as well. Therefore, when maintenance is taken into consideration, it is desirable to dispose a portion of the inverter on the ground.